Arsenic contamination of groundwater has been recognized as a global public health problem since the 1990s, especially in developing communities without access to conventional water treatment facilities, with an estimated 70 million affected. Health issues were initially observed due to skin lesions and other health effects of large populations in Bangladesh and West Bengal, India, which was correlated to high levels of arsenic in their drinking water. Bangladesh, India, and Nepal have experienced a massive epidemic from arsenic groundwater contamination, partially due to installed tube wells to collect groundwater and prevent the indigenous populations from using bacteria-contaminated surface water. However, testing has revealed one in five of the tube wells are contaminated by water containing ten to fifty times the arsenic levels considered safe by the World Health Organization. Further, high levels of arsenic are found in water sources around the world, including Argentina, Chile, Canada, Mexico and the US, as seen in FIG. 1. Consumption of water with arsenic levels higher than 10 μg/L has been associated with arsenic poisoning (arsenicosis), though affected areas may see levels as high as 48,0000 μg/L, as seen in Table 1. However, with the exception of drinking water consumers from private and public wells in the Western US (Kumar, Adak et al. 2010), arsenicosis is not a major public health concern in most developed countries. This is because the arsenic removal technologies employed in developed countries, namely precipitation (coagulation-flocculation-sedimentation), lime softening, adsorption, ion exchange, membrane filtration, electrodialysis reversal and electrocoagulation, are not accessible to developing communities.
TABLE 1A compilation of arsenic levels in groundwater around the world.*ArsenicEstimatedConcentrationpopulationCountry(μg/L)ExposedArgentina 1-9,900200,000-2,000,000Bangladesh<1-250057,000,000 at >10 μg/LChile100-1000 400,000China (Mongolia)40-44405,600,000China (Xinjiang)0.05-850  >500Hungary<2-176 400,000India (West Bengal)<1-37006,000,000Mexico (Region Lagunera)8-624400,000Nepal<10-340  3,190,000 at >10 μg/LTaiwan10-1820100,000-200,000  USA (Western USA)48,000—USA (Southern Iowa and34-490 —western Missouri)UK<1-80 —Vietnam (Hanoi) 1-3100>1,000,000*Adapted from (Mandal and Suzuki (2002). “Arsenic round the world: a review.” Talanta 58: 201-235; Nordstrom, D. K. (2002). “Worldwide occurrences of arsenic in ground water.” Science 296(5576); Garelick, et al. (2005). “Remediation technologies for arsenic contaminated drinking waters.” J of Soils & Sediments 5(3): 182-190; Mondal, et al. (2006). “Laboratory based approaches for Arsenic remediation from contaminated water: Recent developments.” J of Haz Mat B137: 464-479)
Arsenic is a metalloid with similar properties to phosphorus. Arsenic oxidizes to form hygroscopic, colorless, odorless As2O3 and As2O5. The principal means of arsenic dispersion through nature is via water, and varies from locations based on soil and arsenic forms. Arsenic has been attributed to changes in respiratory, gastrointestinal, hematopoietic, and cardiovascular systems. Because of the similarities between arsenic and phosphorus, arsenic can substitute in place of phosphorus in some biological reactions, making it poisonous. Particularly, consumption of arsenic-contaminated water may enter the metabolic citric cycle, inhibiting succinate dehydrogenase and preventing ATP production. Arsenic poisoning is cumulative and symptoms include nausea, vomiting, stomach aches, diarrhea, and delirium, skin lesions and cancers, cancers of the kidneys, bladder, lungs and liver, hyper-pigmentation and hyperkeratosis, and gangrene (“black foot disease”). Consequently, considerable effort has been put forth to reduce and mitigate exposure to arsenic-contaminated drinking water in affected communities.
Many technologies exist to remove arsenic from drinking water. These include precipitation (coagulation-flocculation-sedimentation), lime softening, adsorption, ion exchange, membrane filtration, phytoremediation, electrodialysis reversal and electrocoagulation. The strong affinity of iron compounds for arsenic is exploited in adsorption and precipitation systems. Of these two, precipitation (coagulation-flocculation-sedimentation or CFS) is popular because it is a relatively simple technology with low capital cost and depends on ferric salts which are common, easily found and relatively inexpensive. In CFS, the arsenic is removed by several mechanisms: the conversion of dissolved arsenate and arsenite to solid ferric arsenate and ferric arsenite phases, as well as adsorption and entrapment in hydrous ferric oxide phases. Typically ferric chloride or sulphate is used. The ferric salt facilitates floc formation, which leads to settling and sedimentation of the solid arsenic-bearing precipitate. However, ferric arsenate is relatively soluble (Tenny and Adams, Ferric salt reduce arsenic in mine effluent by combining chemical and biological treatment. Environ Science and Engineering. January 2001), and takes significant amounts of time to precipitate from solution. In some instances, coagulant aids are used to enhance floc formation by producing larger, denser flocs, which settle faster and shorten the sedimentation time. Further, the use of coagulant aids also decreases the dosage of coagulant needed. The coagulant aids are typically synthetic organic polyionic polymers, e.g. MAGNAFLOC, that function as bridges between microfloc particles (source). Recently, there has been an interest in using natural coagulants, flocculants and coagulant aids. The advantages of natural flocculants are that they are generally less toxic, more environmentally friendly, biodegradable and in some instances cheaper and more abundant than synthetic flocculants (Ghebremichael, et al. A simple purification and activity assay of the coagulant protein from Moringa oleifra seed. Water Res. 2005, 39: 2338-2344; Ghebremichael, et al. Combined natural organic and synthetic inorganic coagulants for surface water treatment. J Water Supply: Res and Tech-Aqua. 2009, 58(4): 267-276; Ghebremichael, et al. Moringa oleifra: a natural coagulant, adsorbent and filter aid. Water quality Technology Conference, Cincinnati, Ohio Nov. 16-20, 2008; Ghebremichael, et al. Moringa oleifera for simultaneous coagulation and disinfection in water purification. 57 Can Water Resources Assn Ann Congr., Montreal, Qc, CA, Jun. 16-18, 2004).
Most plant species produce an exopolysaccharide, a polymer of mono- and polysaccharides and proteins bonded by glycosidic bonds, referred to as mucilage. Plants secrete the substance to slow water loss, aid germination, and store food. The tuna cactus (Opuntia ficus indica) mucilage produced by the flattened pads of this cactus was of particular interest, of complex carbohydrates forming a neutral mixture of approximately 55 high-molecular weight sugar residues composed basically of arabinose, galactose, rhamnose, xylose, and galacturonic acid. The mucilage has the capacity to interact with metals, cations and biological substances. Importantly, mucilage swells in water but is insoluble. As such, the substance has the potential to precipitate ions, bacteria and particles from aqueous solutions. Further, the material has unique surface active characteristics, making it an ideal candidate for enhancing dispersion properties, creating emulsifications, and reducing surface tension of high polarity liquids.
This creates a need for accessible technologies, which are relatively inexpensive, reliable, robust, made from available, abundant materials for removing water contaminants, such as (As(V)), and which require little or no fossil fuel energy to work.